A typical photovoltaic cell is a diode with an n-p junction, the junction being very shallow and parallel to the surface. When illuminated, photons of energy hν greater than the forbidden bandwidth Eg of the semiconductor are absorbed and create electron-hole pairs. The minority carriers as generated in this way (holes in the n zone and electrons in the p zone) are collected by the n-p junction. This results in a current Iph, that flows from the n region towards the p region. Metal contacts are made on the surface of the n-type zone (emitter) and on the rear face of the p-type zone (base) in order to collect the current.
The market for photovoltaic electricity is dominated by photovoltaic cells of the n-p (or p-n) junction type made on crystallized silicon. It is always necessary to seek a maximum value for the density of the photocurrent generated under given illumination. This amounts to maximizing collection of minority carriers generated by the useful fraction of the incident radiation (photons of energy hν>Eg). Several techniques are used for this purpose. By way of example, mention can be made of: increasing the thickness of the base in order to enable long-wavelength photons close to infrared to be absorbed (but to the detriment of silicon consumption); reducing the depth of the junction in order to encourage the absorption, in the base, of short-wavelength photons, close to UV; depositing an anti-reflection layer on the illuminated face in order to improve collection of incident radiation; or indeed reducing recombination processes at the interfaces (front face, rear face, contacts).
A final technique is texturing the surface. This technique, e.g. described in the article by J. Nijs, J. Szlufcik, J. Poortmans et al., published in IEEE Trans. Electron Devices 46 (10) (1999) 1948, consists in forming surface relief, in other words in texturing the surface so as to form pyramids. FIG. 1 shows the principle on which this technique operates. The face 10 of the silicon layer 12 that receives light is made up of an array of quasi-identical and adjacent pyramids 14 (represented by triangles in section) having side faces forming an angle of about 45° relative to the bases of the pyramids. An incident light beam 16 normal to the surface gives rise firstly to a refracted beam 18 which is absorbed in the layer 12, and secondly to a reflected beam 20. The reflected beam strikes the adjacent pyramid and gives rise firstly to a reflected beam 22 going away from the silicon layer and is therefore lost, and secondly to a first refracted beam 24 followed by a second refracted beam 26 that is absorbed by the layer. The relief thus increases the overall efficiency of the photovoltaic cell. Specifically:                the effective coefficient of reflection for light on the entry face is reduced, in particular when the incident light presents a large component of diffuse light; and        the angle of inclination of light rays that propagate in the base relative to the macroscopic surface of the photovoltaic cell is greatly increased, which has two consequences: firstly there is an increase in propagation distance within the base of the semiconductor; and secondly there is an increase in the coefficient of reflection of light on the rear face of the semiconductor. These two effects increase the probability of long-wavelength photons being absorbed. Consequently, they contribute to increasing the density of the photocurrent and the open-circuit voltage of the solarcell compared with a solarcell in which the silicon layer has faces that are plane.        
The presence of such texturing becomes very important for maintaining high conversion efficiencies when it is desired to reduce drastically the thickness of the silicon layer typically from around 300 micrometers (μm) or 350 μm (conventional photovoltaic cells), down to less than 100 μm, or down to 50 μm (or even less), in order to reduce the cost of such devices. In this range, and while the coefficient of reflection on the rear face is typically less than 0.6 (normal incidence on the rear face), a large fraction of the spectrum of the incident radiation propagating close to the normal at the surface is not absorbed in the thickness of the material unless special precautions are taken.
In a first known texturing technique, that applies only to single crystal silicon plates having their surface close to the (100) crystal plane, texturing is performing by chemically etching the surface using a solution of KOH and isopropanol. Such etching is highly anisotropic and specific to the (100) crystallographic face, and enables very regular pyramids of micrometer size that are inclined at 45° to be obtained on the macroscopic surface. However, that technique is much less effective when it is applied to plates of polycrystalline silicon, as are being used more and more for reasons of cost.
Under such circumstances, other techniques have been tried. Nevertheless, those techniques rely on isotropic etching, i.e. etching that is assumed to attack all grains under similar conditions: chemical or electrochemical etching using an acid medium (described in the article by V. Y. Yerokhov, R. Hezel, M. Lipinski, R. Ciach, H. Nagel, A. Mylyanych, P. Panek, Solar Energy Materials & Solar Cells 72 (2002) 291-298), reactive ion etching (RIE) in a gas, e.g. using a plasma containing chlorinated species (described in the article by S. Fujii, Y. Fukawa, H. Takahashi, Y. Inomata, K. Okada, K. Fukui, K. Shirasawa, Solar Energy Materials & Solar Cells 65 (2001) 269-275).
Another known technique relates to mechanical etching described in the article by F. Duerinckx, J. Szulfcik, J. Nijs, R. Mertens, C. Gerhards, C. Markmann, P. Fath, G. Willek, High efficiency, mechanically V. textured, screen printed multicrystalline silicon solar cells with silicon nitride passivation, Proceedings 2nd World Conference on PV Solar Energy Conversion, 1998. Mechanical etching consists in forming relief mechanically, e.g. an array of mutually parallel grooves or pyramids directly on the surface of the layer of silicon using mechanical tools such as a diamond grindwheel. Nevertheless, that operation considerably disturbs the structure of the silicon over a thickness of about 10 μm, thereby having the effect of inducing defects throughout the volume of the silicon following the heat treatments to which the silicon is subjected subsequently. In addition, mechanical etching is slow and expensive and therefore industrially ineffective.
Each of those techniques presents limitations that are severe, either in terms of cost (electrochemical etching, plasma etching, and mechanical etching), or in terms of effectiveness (acid chemical etching). Several of them are not applicable to plates that are very thin, of thickness smaller than 300 μm, which are generally very fragile, given the manipulations and/or the mechanical stresses they involve. That applies to mechanical etching and to some extent to electrochemical cleaning (manipulations). The method of the present invention does not present the above drawbacks.